1. Field of the Invention
The present invention relates to an implantable medical device, and more particularly, to an apparatus for connecting conduits in a body, such as forming an arterio-venous (AV) access fistula or arterial or venous bypass, and methods of constructing and deploying the same.
2. Description of Related Art
It is a common surgical procedure to implant a vascular graft between an artery and vein to create a shunt for dialysis access. Typically, these grafts comprise a biocompatible conduit, either autologous (vein or artery) or synthetic, which can withstand repeated cannulation during the course of prolonged therapy.
Among the preferred synthetic tubes for artery-vein (AV) access are any of a variety of expanded polytetrafluoroethylene (PTFE) vascular grafts sold by W. L. Gore & Associates, Inc., Flagstaff, Ariz., under the trademarks GORE-TEX.RTM. and DIASTAT.RTM..
A similar surgical procedure is required when an artery or vein becomes occluded. A synthetic graft or autologous vein or artery is sutured to the vessel both upstream and downstream of the occlusion so as to conduct the flow of blood around the blockage. The most common synthetic grafts used in these applications are those made from polyethylene terephthalate material or expanded PTFE material.
Expanded PTFE vascular grafts exhibit numerous properties that make them particularly suited for the above applications, including being highly biocompatible, being relatively easy to handle and suture, and having suitable flow surface characteristics.
Despite the good performance characteristics of existing synthetic vascular grafts for AV access, the majority of the AV grafts fail in a relatively short period of time after implantation, thereby requiring surgical intervention. The primary cause of complications and failures with synthetic grafts used for AV access has been attributed, in large part, to venous anastomotic stenosis, or anastomotic hyperplasia. Anastomotic stenosis is a common failure mechanism of grafts used for arterial bypasses, as well.
Although the exact mechanism or mechanisms resulting in anastomotic stenosis are not fully understood, theories exist regarding its etiology. Stenotic anastomoses are thought to be a result of native vessel response to injury caused by surgical trauma, unnatural shear stresses, mismatch of mechanical properties between the graft and the native vessels, and/or disturbed hemodynamics. Additionally, disturbed flow may cause deposition of formed blood elements in the perianastomotic region, resulting in the release of growth factors that are chemotactic and/or mitogenic to fibroblasts and smooth muscle cells. The factors induce these cell types to migrate into the intima of the native vessel and proliferate, causing the anastomosis to become stenotic. The presence of suture or staple holes in the anastomotic region not only provide pathways through which cells can migrate, they are also areas of local injury which can lead to the release of growth factors. Through these mechanisms, suture or staples used in the construction of an anastomosis also increase the potential for anastomotic stenosis.
The present inventors believe that an improved implantable junction device designed to minimize the stenotic consequences of these effects should have the following features: 1) appropriate hemodynamics to minimize turbulence and inappropriate shear stresses on native arteries or veins; 2) implantation means that do not require sutures or staples that penetrate through to the luminal surface of the blood vessel or the device; 3) when used to join two streams of blood, isolation of the native vessel from direct impingement by the incoming stream of blood; 4) sufficient "impact isolation distance" from where the incoming stream of blood impinges to the end of the device; and 5) sufficient "cellular isolation distance" whereby cut and traumatized vessel tissues are isolated from the lumen so that cells are limited from migrating into and proliferating onto the luminal surface of the device or the native vessel. It is believed that an improved anastomotic junction incorporating the above features will address many of the issues related to anastomotic stenosis as a complication and failure mechanism of vascular grafts.
A number of devices have been proposed to provide a better junction in a blood vessel. For instance, T-shaped grafts that can be inserted into a severed end of a blood vessel have been proposed for various surgical procedures. U.S. Pat. No. 3,683,926 to Suzuki and U.S. Pat. No. 4,503,568 to Madras teach a number of different junctions for joining separated blood vessels, including T-shaped and Y-shaped junctions, generally referred to "Y-shaped" herein, that can attach together multiple blood vessels. While these devices may eliminate direct flow impact against the vessel wall at the junction, these devices have a number of other problems. First, surgeons generally attempt to preserve host vessels by avoiding unnecessary cutting and removal of sections of vessels. Both Suzuki and Madras teach complete severing of the blood vessel, which requires removal of a vessel segment to avoid redundancy. Second, excessive proliferation of normal cells has been observed at the interface between the severed blood vessel and the junction device. This excessive proliferation results in stenotic formations over a relatively short time frame. Another deficiency with the Suzuki device is that Suzuki's junction is readily soluble in blood. This is likely to lead to questionable anastomotic integrity and only temporary protection of the native vessel from the impingement of the incoming stream of blood. Given these deficiencies, it is not surprising that devices such as those taught by Suzuki and Madras are not widely used.
Some of these complications might be addressed if a junction device could be inserted into a blood vessel without requiring a complete severing of the blood vessel. Devices of this kind are disclosed in the U.S. Pat. No. 4,512,761 to Raible, U.S. Pat. No. 5,443,497 to Venbrux, and U.S. Pat. No. 5,456,712 to Maginot. A similar concept is disclosed in U.S. Pat. No. 4,230,119 to Blum, wherein Blum teaches a micro-hemostat that permits blood flow during vascular surgery. All of these devices are based on the concept of providing some junction device that is installed within the natural blood vessel. Unfortunately, such devices do not contain all of the features believed necessary to minimize anastomotic stenosis.
The Raible patent teaches a junction device for providing blood access external to the body. This device requires sutures at the device inlet, which, as has been noted, are believed to be undesirable. Similarly, the Maginot patent teaches using sutures or staples that pass from the outer surface to the luminal surface to attach the two components of his device. While Venbrux provides for sealing the penetration through the native vessel, he provides neither for an "impact isolation distance" nor for a "cellular isolation distance."
Accordingly, it is the primary purpose of the present invention to extend primary patency of an AV fistula or blood vessel bypass through the use of an improved junction device, where primary patency is defined as the length of time a graft remains patent without medical intervention.
It is a further purpose of the present invention to provide a medical junction device that can be relatively easily, uniformly, and consistently installed by a surgeon.
These and other purposes of the present invention will become evident from review of the following specification.